Testing a power supply without hooking up its intended load

Question

I have a PSU that I would like to verify and see if it is outputting all of the correct voltages.

It's an old computer PSU from 1990 with a specific form factor that cannot be easily replaced with a new one.

The PSU will only switch on with a certain load attached to it. Usually it is sufficient to hook up a hard drive or an old floppy drive in order to get a computer PSU to start properly. Without load it will either not start, or try to switch on but make a clicking noise. Without load it is also not possible to measure any output voltages.

I am always a bit reluctant to hook up the actual intented load on to it (in this case a mainboard + hard drive + floppy drive), in fear of risking to damage the mainboard should there still be an issue with the PSU.

What is the best way to simulate such a load ? I was thinking about hooking up some resistors on certain power lines (The PSU also exposes -12V) but I am a bit unsure on how to calculate the resistor values (actual resistance but also wattage)

What would be a safe / meaningful way to simulate some load and make the PSU think it is hooked up to the mainboard, so I can verify that the voltages are OK before hooking up the PSU to the mainboard.

Answer 1

If you want to test this on your own, a few ceramic power resistors should be enough. Calculate the load with Ohm's Law. You obviously want a high load, so choose a low resistance. The specific value doesn't matter as long as the load is below the max current for the rail AND for safety is within the rated wattage of the resistor. A 10 ohm, 15 watt resistor for the 12V rail is good for example, drawing 1.2 amps.

Ideally you could add a 12V fan across your resistors to help manage the heat. A few fans would also provide a load for testing.

Typically you only need to load the 5V or 12V rail for all the rails to be stable or for the PS to turn on. It's normally whichever rail is the higher capacity one. Older PSUs tend to be 5V while newer are 12V.

That said, you can buy ATX load testers online at the usual stores that already have everything you need to load test an ATX power supply.

Answer 2

I'm not too familiar with computer PSUs, but I can offer a few things that I think might help.

1) Look at the specifications for your intended load. In a datasheet or specsheet there should be a power draw spec or at least an operating voltage and a range of possible current draws. This allows you to look at the worst case(most power absorbed by load) and best case(minimum power absorbed by load). After doing so, you could create a resistor network on the output terminals you're interested in that draw the same power. It's important that you choose resistors suited for that power draw, otherwise they're going to overheat.

2) If you want to do it empirically, you could connect a network of potentiometers (each with large values) in series, and you can connect them so that they are all at the highest impedance at first. This would essentially simulate an open circuit, so you would have close to zero power draw. After doing so, you can very gradually, one by one, lower the resistance of each pot (from highest impedance to a short) until you can see that the power supply is switching on. It's important that you never try to draw more power than allowable by the PSU though, as this will damage it, so make sure to check the spec on max power for your PSU.

I'm making these assumptions based on my usage with general power supplies on lab benches. I'm not sure about the specifics of computer PSUs, but it's strange to me that the power supply refuses to turn on without a load, seeing as an open circuit shouldn't damage the supply in any way I believe. This is a reach, but it sounds like your power supply is somehow shorting without a load (maybe internally) that's causing it to not turn on, maybe as a short circuit protection mechanism. This probably isn't the case seeing as it works with certain loads. Hope this helps.

Answer 3

my experience is with ATX pc power supplies, different brands (models even) might vary. but the load doesnt matter. if its enough for it to turn on, it cant really tell what exactly its trying to power, it just knows SOMETHING is pulling current so it should stay on. ive got one that i use to power a 3d printer that i trick into staying on by using a 5v usb led strip that i cut the plug off and twisted the wires together. also older supplies do their sensing on either the 5v or 3.3v line, some newer ones on the 12v but shortly after they stopped needing minimum loads for the most part, so you could probably just hook up an old cell phone or ANY old device that has usb input to the 5v line and short the black and green wires